I want to try another provider, but my family (partner, kids, grandparents) stores pictures in Google Photos, shares docs from google drive, etc. It is all the extra stuff I get with Google that feels daunting to me, I'd love to switch for email and calendar.
You can just have a gmail account you don't use, and in fact even if you have your own domain, if you're willing to pay for Google separately, you can have a Google Workspace account for your custom domain and your e-mail for that domain at Fastmail and just not point any MX records at Google. It works fine - it's what I do as an artifact of having used them for e-mail and moved but not having bothered to untangle my other stuff (yet; but it's been years).
The only thing that's a bit of a hassle is the calendar. It also mostly works, since you can add your Fastmail calendar to Google Calendar, but Google is slow to sync.
And you can move your email but leave an unused account in existence if something really really needs a G login. Not great, but doable and some time later you might find out it isn't needed after all.
